powerpc/ps3: Use str_write_read() in ps3stor_read_write_sectors()

powerpc/ps3: Use str_write_read() in ps3stor_read_write_sectors()

Remove hard-coded strings by using the str_write_read() helper function
and silence the following Coccinelle/coccicheck warning reported by
string_choices.cocci:

  opportunity for str_write_read(write)

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250819165829.714344-2-thorsten.blum@linux.dev